Semiconductors, such as integrated circuits, are formed on wafers which are then cut into dice or chips that individually may be mounted on substrates. Typically, the substrate is electrically and thermally conductive, with mounting providing both good electrical and good thermal conductivity between the die and the substrate.
Known methods for attaching a die to an electrically and thermally conductive substrate include employing a solder or eutectic alloy such as a gold-silicon alloy, employing a spreadable adhesive consisting of a heat-curing epoxy resin composition filled with fine metal particles; and employing an electrically and thermally conductive adhesive composition which comprises an adhesive containing fine metal particles or a deformable metal foil. See, for example, U.S. Pat. No. 4,606,962.
The metal eutectics are used most specifically in the area of power devices, to provide a metallurgical interface between a silicon die and the heat-sinking metal or ceramic substrate with optimum thermal and electrical conductivity. This technique is relatively successful for smaller devices, but is not desirable for use with very large dice, which may approach 0.6 inch on a side. The differential coefficients of expansion of the substrate and the silicon die can result in a larger die cracking under the stresses imparted by a very rigid bonding medium, and may result in its subsequent failure.
Epoxy-silver compositions are extensively used for commercial die-bonding, as they provide an often suitable compromise in terms of cost, stress-relief and electrical/thermal conductivity. However, epoxy-silver compositions have the following undesirable characteristics: the lack of uniformity of dispersion of silver particles within the adhesive composition, the lack of uniformity of the mixture of two component systems (epoxy and curative), the lack of coplanar (die/substrate) maintenance during cure, the presence of resin bleed into the surrounding area or onto the die's active surface prior to curing, and unsuitably low shear strengths, as measured by the military standard, MIL-883C.
The upper temperature performance of these organic adhesives is typically less than that of the metallurgical bonding materials, and is related to the glass transition temperature (T.sub.g) of the composition. The usual way to improve this upper temperature performance is to use a very highly cross-linked adhesive system, thus raising the T.sub.g. This approach, however, contributes to increased stress on a die, an undesirable characteristic as discussed above.
A superior die-attach system is described in copending application U.S. Ser. No. 24,997-filed on even date. It provides an adhesive composite having a resilient structured conductive layer with an adhesive layer on each side. This adhesive composite provides electrical and thermal conductivity between a die and a substrate and through its resiliency can maintain connection when exposed to extremes in temperature.
It is therefore highly desirable to provide an adhesive composition capable of being used in such an adhesive composite which is capable of rapidly curing to effect bonding of dice to a variety of substrates, exhibits little or no tack and has a suitable shelf life at or below room temperature, can be readily adhered to and diced to precise size with a silicon wafer, and exhibits none of the bleeding and migrating characteristics common to adhesive/metal pastes discussed above. It is further desirable that the adhesive composition exhibit superior shear strength and adhesion to a multiplicity of substrates and surfaces, has excellent resistance to degradation due to thermal excursions as high as 350.degree. C. and greater and exhibit improved resistance to moisture penetration and degradation of adhesion in aging tests conducted over a range from -65.degree. C. to 150.degree. C.